If the radiator isn't boiling over from underneath the radiator cap, I doubt it needs a new radiator cap. But if you decide to replace it, make sure to get an 18 lb. model.
You flushed the radiator but did you check it with a flashlight inside underneath the radiator cap to see if it is clogged up any with hard water/mineral deposits? Has it tap water ever been added to the radiator when mixing up the coolant? Tap water can cause the radiator to clog up with disolved minerals, only distilled water should ever be added to a radiator (or the battery).